A semiconductor unit that is provided with a semiconductor light-emitting device is commonly known. Such a semiconductor unit is produced by mounting a semiconductor light-emitting device on a submount 103 as shown in FIG. 5. FIGS. 5 and 6 are schematic cross-sectional views explaining a method of producing a conventional semiconductor unit. A method of producing a conventional semiconductor unit is explained below by referring to FIG. 5.
As shown in FIG. 5, in a method of producing a conventional semiconductor unit, first, a submount 103 for mounting a semiconductor light-emitting device on it is prepared. The submount 103 comprises:                (a) a ceramic substrate 104;        (b) a bi-layer 105 formed on the substrate 104, the layer being composed of a layer comprising titanium (Ti) and a layer comprising platinum (Pt) (Ti/Pt bi-layer 105);        (c) a gold (Au) layer 106 as an electrode layer formed on the Ti/Pt bi-layer 105;        (d) a solder-protecting barrier layer 107 comprising platinum (Pt) formed on the Au layer 106; and        (e) a solder layer 108 comprising gold (Au)-tin (Sn)-based solder formed on the solder-protecting barrier layer 107.In the submount 103, the Ti/Pt bi-layer 105, the Au layer 106, the solder-protecting barrier layer 107, and the solder layer 108 may be formed by using a conventional layer-forming process, such as a vapor deposition method, a sputtering method, or a plating method, or a patterning process, such as a photolithographic method or a metal mask method.        
After the submount 103 is prepared as shown in FIG. 5, the solder layer 108 of the submount 103 is heated and melted. A detector 200 detects by image processing whether or not the solder layer 108 is melted. More specifically, before the melting, the solder layer 108 reflects a large amount of light. Accordingly, the detector recognizes the color of the solder layer 108 as “white” by using the binarization in image processing. After the melting, the solder layer 108 reflects a small amount of light. Then, the detector recognizes the color of the solder layer 108 as “black.”
As shown in FIG. 6, when the detector 200 recognizes the color of the solder layer 108 as “black,” a laser diode 102 as a semiconductor light-emitting device is mounted on a predetermined position on the solder layer 108 (a die-bonding step is performed). Subsequently, the solder layer 108 is cooled and solidified. This process securely bonds the laser diode 102 to the submount 103 with the solder layer 108. Then, the back side of the submount 103 is securely bonded to a heat sink (not shown in the drawing) with solder or the like to complete the production of a semiconductor unit provided with a semiconductor light-emitting device.
The conventional semiconductor unit produced by the process shown in FIGS. 5 and 6 has the following drawback. When the detector 200 recognizes the color of the solder layer 108, if the solder layer 108 has a rough surface, the surface of the solder layer 108 reflects the light diffusely. Consequently, the detector 200 cannot obtain sufficient incident light, so that it recognizes the color of the solder layer 108 as “black.” As a result, either the die-bonding equipment suffers an error and stops functioning or the laser diode 102 is pressed against the solder layer 108 before it is melted without successfully bonding the laser diode 102 to the submount 103.